1. Field of the Invention
The present invention concerns a method of forming a color filter for a solid-state image sensing device having on-chip color filter and, more in particular, it relates to a method of forming a color filter used in a solid-state image sensing device for forming a color filter made of a resist material on a planarized layer comprising a thermosetting resin formed on the surface of the solid-state image sensing device.
2. Description of the Related Art
A solid-state image sensing device includes such a device having a color filter formed on the surface of a solid-state image sensing device. Since the solid-state image sensing device has a remarkable surface unevenness, a planarized layer comprising a thermosetting resin is generally formed on the surface of the solid-state image sensing device and then the color filter is formed on the planarized layer. A resist material is often used for the material of the color filter. That is, the color filter is formed by coating a resist, applying exposure and development and then applying a heat treatment at a predetermined temperature for improving the adhesion with the underlying layer.
By the way, in the prior art, micro-cracks are often formed in a planarized layer made of the thermosetting resin in the solid-state image sensing device. Such micro-cracks cause deterioration of optical image sensing characteristics of the solid-state image sensing device. Such micro-cracks include those giving disadvantages in the initial stage and those resulting in disadvantages as an aging effect and both of them are innegligible since they deteriorate the characteristics of the solid-state image sensing device. Actually, failures caused by the cracks in the planarized layer have been generated so far at a rate as high as from 40 to 80%.
In view of the above, the present inventor has investigated the cause of the cracks, and has been found that thermal stresses induced by a heat treatment for the color filter after forming the planarized layer is attributable to the occurrence of the micro-cracks in the planarized layer. Further, as a result of proceeding the study for reducing the thermal stress, it has been found that the heat treatment temperature upon forming the color filter is excessively high. The present invention has been accomplished as a result of such studies.